The report in today’s Guardian that “becoming king will not silence Prince Charles” is bound to arouse a degree of trepidation. Since 1910 we have become used to monarchs who with conscientious self-effacement have for most of the time kept their political opinions to themselves. The present Queen has followed the example set by her father, George VI, and he in turn learned much from his father, George V. The unwillingness of Edward VIII to subject himself to this kind of self-denial meant he could not long remain King, and indeed felt a strong impulse to get out of being King. He was made to abdicate because he had chosen an unsuitable wife, but he also had unsuitable views on foreign policy which would have caused increasing friction with ministers.

Walter Bagehot, a journalist of genius, in 1867 supplied a definition of the monarch’s rights on which no one has yet managed to improve. He wrote:

“To state the matter shortly, the sovereign has, under a constitutional monarchy such as ours, three rights – the right to be consulted, the right to encourage, the right to warn. And a king of great sense and sagacity would want no others.”

But when Bagehot wrote this, he was trying to work out what the limits were to interference by the monarch in the policy adopted by the Government on the great political questions of the day. He did not have in mind the kind of issues which are so important to Prince Charles. The Guardian’s report says:

Prince Charles is ready to reshape the monarch’s role when he becomes king and make “heartfelt interventions” in national life in contrast to the Queen’s taciturn discretion on public affairs, his allies have said.

In signs of an emerging strategy that could risk carrying over the controversy about his alleged meddling in politics into his kingship, sources close to the heir say he is set to continue to express concerns and ask questions about issues that matter to him, such as the future of farming and the environment, partly because he believes he has a duty to relay public opinion to those in power.

“He will be true to his beliefs and contributions,” said a well-placed source who has known him for many years. “Rather than a complete reinvention to become a monarch in the mould of his mother, the strategy will be to try and continue with his heartfelt interventions, albeit checking each for tone and content to ensure it does not damage the monarchy. Speeches will have to pass the following test: would it seem odd because the Queen wouldn’t have said it or would it seem dangerous?”

We need, perhaps, a new Bagehot, who can devise a phrase which covers “heartfelt interventions”. The dangerous word here is “heartfelt”. Prince Charles is immensely sincere in his views about questions such as farming and the environment. His idea of being a conscientious monarch includes speaking up on such matters. It is, he believes, his duty to speak up. He can do no other.

The problem with this is that it could clash with his duty to remain above politics. One of the greatest services our monarchy performs is that it occupies the space which a dictator would need to occupy. In this sense, the monarch remains a vital guarantor of our constitution and our liberties. But the monarch does this by not being a politician.

We live in a parliamentary monarchy. What happened in 1649, 1688, 1714 and 1936 confirmed and demonstrated this. So the monarch has to be careful not to defy Parliament, or any substantial body of public opinion.

These abstract doctrines are always to some extent subject to the mood of the times. When George VI leant public support to Neville Chamberlain after Munich, few people objected, because Chamberlain was at that point seen as the man who had brought peace. But a monarch who defies the mood of the times will soon be in trouble: a point that was just as true when monarchs still had political power as it is now.

So Prince Charles will need to proceed with considerable care. Conscientiousness will not be a sufficient defence if he campaigns for policies to which some considerable body of public opinion objects. Sometimes the conscientious course will be to keep his opinions to himself.